Commercial electricity production began yesterday at what Sempra Energy calls the largest solar plant of its kind in North America, a unit that will power homes and businesses in Northern California for at least the next 20 years.

The 10-megawatt plant owned by Sempra subsidiary Sempra Generation covers 80 acres of desert in Boulder City, about 40 miles south of Las Vegas. It's next to a 480-megawatt gas-fired plant, also owned by Sempra Generation.

The plant was built in six months and is the first of many, said Michael Allman, Sempra Generation's chief executive.

“Our goal is to be the first company to own and operate 500 megawatts of solar power in the world,” he said. “We expect that most of it will be in the Southwest.”

San Diego-based Sempra said it has sold the plant's output until 2029 to San Francisco's Pacific Gas & Electric, which has about 15 million customers.

When running at peak capacity, the solar plant will power about 6,400 customers, Sempra said. Output of solar plants varies with the time of day and how brightly the sun is shining.

Overall, the 23.2 gigawatt-hours the plant is expected to generate a year will power about 3,400 customers, PG&E said.

The plant uses thin-film technology, which produces less electricity per panel than traditional silicon-based photovoltaic power but is cheaper to build.

Photovoltaic power converts light directly into electricity. Other solar technologies rely on heat from the sun to power generators.

Sempra Generation, a sister company of San Diego Gas & Electric Co., sells its power independently.

SDG&E has a contract to buy up to 900 megawatts of solar power from Stirling Energy Systems, an Arizona company that is planning to use engines mounted on mirrored dishes in an unprecedented effort to produce power in Imperial Valley.

PG&E is expected to derive about 13 percent of its power this year from renewable sources, such as solar and wind, said PG&E spokeswoman Jennifer Zerwer.

Investor-owned utilities in California are under state mandate to get 20 percent of their power from such sources by 2010.

The plant was built, designed and outfitted by First Solar of Tempe, Ariz., which until recently sold its products to European customers.

First Solar's largest plant is a 40-megawatt plant in Germany. The company also provided solar panels for a plant on the roof of a San Bernardino County warehouse.

Big plants are more economical than rooftop installations, Allman said. They can be put in places like the desert, where the sun is bright and land is cheap. And installers work assembly-line style, so they are more efficient.

The Boulder City plant, for instance, consists of 167,000 individual solar modules.

None of the companies involved would discuss how much the plant costs or the price Northern California customers will pay for its electricity.

But Allman said it is more expensive than the power produced next door by burning natural gas.